Structural alterations and expression of immunoglobin (Ig), T-cell receptor (TCR), and various growth affecting genes are studied in normal, "premalignant", and malignant tumors and cell lines. A. Our focus this year has been a study of the mechanism of formation of hybrid interlocus Ig and TCR genes. We have shown that hybrid genes are formed by site specific recombination between variable segments from one locus (for example TCR gamma) and joining segments from another (TCR beta). We have demonstrated that such events occur in the peripheral T-cell of all normal individuals but are 100 times more frequent in the peripheral T-cell of patients with ataxiatelangiectasia (AT). These hybrid genes 1) affect and alter the repertoire of immune receptor (Ig and TCR) diversity 2) suggest that an underlying defect in AT may be chromatin "hyperaccessibility" and 3) provide a possible screening test for people at an increased risk for the development of lymphoid specific chromosomal translocations and therefore lymphoid malignancy. B. RNA-RNA tissue in situ hybridization. The expression of individual cells within tissue sections from lymph node biopsies and peripheral blood form patients with lymphoid malignancies have been analyzed with immunoglobulin, T cell receptors, and oncogene probes. This technique refines the analysis of such tissue to the point where the unique gene expression of one cell in hundreds of thousands can be identified. Furthermore we have developed a technique of direct RNA sequencing the allows us to utilize tumor specific gene expression as a tumor specific marker providing a potentially powerful and sensitive means of cancer diagnosis and staging. C. Gene and transcript mapping. We have localized numerous genes of interest to specific regions of human chromosome by in situ hybridization. Refinement of this technique using biotinylated probes is increasing the sensitivity and specificity or our work and allows us to study genotopography in interphase nuclei.